Electrostatic voltages are known to provide a threat to electronic components. Electronic components can be damaged when a charge stored on a person, on a piece of equipment, or on a surface rapidly dissipates or discharges through an electronic component or assembly.
An accumulation of extra electrons occurs naturally when two predominantly insulated surfaces are rubbed together. For example rubbing ones feet across the carpet can easily cause a voltage of thousands of volts to accumulate on a person. In this instance the person is insulated from a ground or from a conductive surface by the carpet, as the person rubs their feet they accumulate extra electrons. Since each electron has a charge typically measured in Coulombs (C) the more electrons accumulated on the person, the higher the charge a person carries. The person is storing charge just like a capacitor stores charge, by accumulating electrons. Such accumulated charge is referred to as a static charge because it is not moving relative to the person or surface on which the charge is accumulated.
If a person is significantly charged and touches a door knob a spark frequently jumps between the person and the door knob. This event discharges some or all of the accumulated charge on the individual and into the door knob through an electrostatic discharge event.
The charge carried by a single electron is e, and e=−1.602×10−19 C (Coulombs). The number of Coulombs (C) stored on a capacitor, typically measured in Farads (F) determines the voltage (V) stored on the capacitor such that 1C/1F=1V. By this formula one Coulomb stored on a 1 Farad capacitor has a voltage of 1 volt.
This formula shows that increasing the number of electrons stored on a surface increases the charge stored on the surface while increasing the voltage. The voltage increases proportionally to the capacitance (the number of Farads) of the surface, where the charge is proportional to the number of electrons stored on or in the surface. Voltages stored statically in such a way are commonly referred to as electrostatic voltages.
Individuals can easily store thousands of volts on their body. If discharged rapidly into an electronic component or assembly the electronic component or assembly may be damaged or destroyed. Some components can be damaged with as little as 50 volts of potential across critical junctions. Similarly static charge can accumulate on pieces of equipment or on surfaces when two surfaces are rubbed together.
Such accumulated charges stored on individuals, equipment, or on surfaces presents a threat to the viability of electronic components and assemblies especially when those components are not in a protective case. Manufacturing environments where electronic components are fabricated or assembled into assemblies are prime examples where static can easily damage the electronics if the buildup of excessive charge is not managed.
Various pieces of equipment designed to mitigate threats from electrostatic voltages such equipment includes wrist straps, foot straps, mats, de-ionizers, and electrostatic dissipation device testers.
Wrist straps, foot straps, and mats are examples of electrostatic discharge devices. Such devices have a resistance optimized to conduct electricity slowly. If the resistance is too low, the device will conduct electricity rapidly, if the resistance is too high, the device will not dissipate a static charge efficiently.
Controlling the series resistance of a person in contact with one or more electrostatic dissipation devices is critical to controlling electrostatic charge accumulation in an electronic manufacturing environment. Typically an operator will wear or be in contact with one or more electrostatic dissipation devices when they are working. Wrist straps and foot straps are typically worn by an individual. Electrostatic dissipative mats are frequently placed on tables in work areas to keep static charge from accumulating on the tables or work areas.
Electrostatic discharge devices prevent charge from accumulating on surfaces or on persons in contact with them by conducting charge to ground. Electrostatic discharge devices also prevent the rapid discharge of accumulated charge when a highly charged object or person contacts them initially by conducting charge to ground slowly.
Measuring the series resistance of a person in contact with an electrostatic discharge device is therefore important to mitigating both charge accumulation and rapid discharge of accumulated charge. In this document the term operator resistance or resistance of the operator is the series resistance of a person in contact with an electrostatic discharge device. Optimally the operator resistance should be between 600 thousand ohms and 3 million ohms.
Electrostatic dissipation device testers typically determine the operator resistance by first applying a known voltage across a known resistance in that is connected in series with the operator resistance. The voltage drop across the operator resistance is then measured and ohms law is used to calculate the resistance of the operator.
Conventional testers capable of testing operator resistance do so by using two analog comparators. The analog comparators compare the voltage drop across the operator resistance to a voltage set by a voltage divider network. Typically one analog comparator is configured to activate when the operator resistance is too low, and the second analog comparator is configured to activate when the operator resistance is too high. Such testers test are limited to reporting pass, fail low (resistance too low) and fail high (resistance too high).
Limitations of testers sold in this market place also include crude displays. For example when performing tests, some testers flash red and green status LEDs simultaneously, and then illuminate a green LED or a red LED indicating that a test has passed or failed respectively. Such displays can confuse operators or cause operators to have questions about the efficacy of the tester.
Conventional ESD testers are also not intelligent, they do not maintain a persistent record of previous tests instead they are limited to sending data to other devices that maintain a data base, and they rely on the use of mechanical buttons or switches to initiate tests.
Conventional ESD testers also cannot be configured by manipulating the tester itself.